Laser polygon scanners, hologon scanners, light valve arrays, and LED arrays are well known devices for selectively exposing or "writing on" photosensitive materials or "photoreceptors". These devices, usually referred to as page scanners, are incorporated in electronic printing systems and are used to record information on the moving photoreceptor by converting serial electronic data into a parallel array of picture elements, or "pixels", line-by-line, on the photoreceptor. Eventually, a full page of information is written as the photoreceptor completes its cycle of motion.
Recently, electro-optic page scanners have been proposed. U.S. Pat. Nos. 4,367,925 and 4,386,827 disclose a page scanner device in which a collimated sheet of light is total-internal-reflected off of the base of an inorganic electro-optic crystal. Selected pairs of parallel electrodes formed on or near the base of the crystal are energized by the application of an external voltage between these electrodes. The fringing electric fields extending into the electro-optic crystal in the region where the light is total-internal-reflected cause a phase change in that portion of the phase front of the collimated beam passing through the affected area. Schlieren optics are used to convert the phase-front modulated beam into a corresponding intensity-modulated pixel pattern representing an entire line of image information. Disadvantages of this device include difficulty in achieving uniformly high coupling efficiency between applied electric fields and optical fields for electro-optic modulation of the incident beam phase front, and expense of the relatively large electro-optic crystals of standard inorganic materials required for such scanners. U.S. Pat. No. 5,052,771 describes the use of thin-film waveguides composed of organic nonlinear optic materials to form an integrated optic waveguide scanner. The device described therein avoids many of the disadvantages associated with prior art electro-optic scanners.
One of the primary disadvantages of these electro-optic page scanner systems is that they require large, expensive lenses or lens arrays. Large lenses or lens arrays must be used because the physical dimensions of the lens or lens array which relays the image of the pixel pattern to the photoreceptor surface must be at least as large as the width of the page to be written. Typically, the page is on the order of 8 to 10 inches wide. This disadvantage can be partially offset by the use of Selfoc lens arrays; however, Selfoc lens arrays suffer from other problems such as severe chromatic aberration and non-uniformity of illumination. A second disadvantage, especially for those page scanner systems which employ incoherent illumination, is low light collection efficiency. In such systems light collection efficiency is limited by the numerical aperture of the lens system. In this respect the Selfoc lens array is no better than a conventional lens systems. Accordingly, it will be appreciated that it would be highly desirable to have a page scanner system with high coupling efficiency and relatively low expense which does not require large or bulky lenses.
The idea of using integrated optics to form compact lens-free devices for generating illumination arrays is not new. Kubota and Takeda in Array Illuminator Using Grating Couplers, Opt. Lett., Vol. 14, Jun. 15, 1989, pp. 651-652 reported a two dimensional illuminator formed by a thick multimode planar waveguide (thickness about 2.5 mm) with a two dimensional array of spaced grating couplers. The grating couplers were designed so that a guided collimated beam from a He-Ne laser source incident upon each grating in the array from inside the glass generated a beamlet that propagates in a direction normal to the waveguide surface. The disadvantage of such a device is that it is passive; no means are provided for individually modulating each beamlet. Furthermore, the total number of beamlets is limited by the relatively large losses associated with each grating.
U.S. Pat. No. 4,776,661 discloses an integrated optic device for performing optical data processing. The device is comprised of a planar waveguide on a substrate and a channel optical waveguide coupled to the planar optical waveguide. A grating coupler along a portion of the channel optical waveguide serves as a means of optically coupling the channel and planar optical waveguides. This device is shown schematically in FIG. 1. Laser diode source 8 endfire couples light into channel guide 6. Grating couplers 10a, 10b formed on channel waveguide 6 are designed to couple a portion of incident beam 12 into beamlets 14a, 14b which are guided in planar guide 4. As can be seen, this device serves to convert light from a single source into a linear array of collimated beamlets. No provision is made for modulating the individual beamlets or for coupling the light from the individual beamlets out of the channel waveguides in such a manner that the light remains collimated in the meridian normal to the plane of the waveguide.
Yet another prior art waveguide illuminator is disclosed in U.S. Pat. No. 4,765,703. This device is an electro-optic beam deflector for deflecting a light beam within a predetermined range of angle. It includes an array of channel waveguides and plural pairs of surface electrodes formed on the surface of a planar substrate of an electro-optic material such as single crystal LiNbO.sub.3. The channel waveguide array is comprised of a main channel and a plurality of branch channels diverging from the main channel on either side of the main channel at respective branching portions. Light from a laser diode is endfire coupled into the main channel at one end of the device. The branch channels are angled with respect to the main channel so as to cover the predetermined range of angle. Each pair of deflector surface electrodes is disposed at a corresponding one of the branching portions of the channel waveguide array to produce an electric field locally at the corresponding branching portions of the channel waveguide, and thereby deflect the guided light beam from the main channel into the corresponding one of the plurality of the branch channels due to the electro-optical effect upon selective application of a voltage between the deflector electrodes. A cylinder lens condenses the light emanating from one of the plurality of branch channels in the meridian normal to the plane of the substrate at the output end of the device.
A principle disadvantage of the device of U.S. Pat. No. 4,765,703 is that the pixels must be addressed in series rather than in parallel. This limits the speed at which the device can operate. A further disadvantage is that light which is to be directed to pixels via corresponding branch channels farther away from the laser diode source must propagate through more electrode branching portions than light which is directed to pixels which are associated with branch channels that are closer to the laser diode source. This will result in non-uniformity of illumination if any light is absorbed or lost at each branching portion. Still another disadvantage of this device is that it is based on the electro-optic effect in bulk single crystal materials such as LiNbO.sub.3. First of all, such bulk single crystals are usually expensive. Secondly, relatively large voltages are usually required to cause electro-optic deflection at each branching portion since the interaction with the electro-optic single crystal is via relatively weak electric fringing fields generated by electrodes on the crystal surface. This means that an array of expensive high voltage drivers would be required to provide the electrical signals for the device. Finally, the device has the disadvantages that all of the deflector electrodes and branching portions are located in series along the main channel. Since a finite length is required for each branching portion, the overall length of the device will be impractically large if the number of required pixels becomes large, for example, in the thousands.
Recently, strides have been made in the area of nonlinear optical organic materials. Penner et al. in U.S. patent application Ser. No. 735,550, filed Jul. 25, 1991 now abandoned, entitled Improved Conversion Efficiency Second Harmonic Generator, disclose means of forming poled noncentro-symmetric organic molecules by means of the Langmuir-Blodgett (LB) technique. Williams has proposed the use of electrically poled noncentro-symmetric organic molecules in guest-host polymer structures. See Nonlinear Optical Properties of Guest-Host Polymer Structures, Quantum Electronics: Principles and Applications, Vol. 1 (1987), edited by Chemla and Zyss. Another means of obtaining nonlinear optical properties is by electrically poling nonlinear molecular species which are co-polymerized with linear organic materials as described in U.S. Pat. No. 4,900,127. Disclosure of specific nonlinear organic molecules with large second order hyperpolarizabilities was made by Ulman et al., New Sulfonyl-Containing Materials for Nonlinear Optics: Semiempirical Calculations, Synthesis and Properties, J. Am. Chem. Soc., Vol. 112, No. 20, Sep. 26, 1990, and also by Williams, Organic Polymer and Non-Polymeric Materials with Large Optical Nonlinearities, Angew. Chem., Int. Ed. Engl. Vol. 23 (1984), pp. 690-703. Other references to organic nonlinear optical media in the form of transparent thin films are described in U.S. Pat. Nos. 4,694,066; 4,536,450; 4,605,869; 4,607,095; 4,615,962; and 4,624,872.
Use of an electro-optic polymer to form a channel waveguide with switchable optical taps is taught by T. E. Van Eck et al. in Complimentary Optical Tap Fabricated in an Electro-Optic Polymer Waveguide, Appl. Phys. Lett., Vol. 58, No. 15, April 1991, pp. 1588-1590. This article describes an "optical railtap" system formed by a main channel waveguide with complimentary directional coupler taps which are disposed at intervals along the main channel rail and which are connected to secondary side channels. The channel waveguides are formed of an electro-optic polymer with electrodes appropriately positioned so that, after poling, the complimentary directional coupler taps can be selectively addressed to couple a portion of the optical energy out of the main rail channel into preselected secondary side channels. Each complimentary directional coupler includes a pair of identical taps, one of which is connected to a dummy secondary side channel. The complimentary directional coupler is designed so that the same portion of optical power is drawn off of the main rail regardless of the voltage applied to the tap; optical power is either directed toward the pre-selected side channel or discarded in the dummy side channel.
Although the Van Eck et al. device is well-suited for use as a means of distribution of laser signals in a network in which there are relatively few, widely-spaced optical taps, it suffers several fundamental drawbacks with respect to applications such as a page scanner which has numerous closely spaced pixels. First of all, since each directional coupler tap is on the order of a millimeter in length, the placement of such taps in series along the rail limits the number and packing density of output channels per unit length of the main rail channel. Secondly, the fabrication tolerance of the two taps in each directional coupler is very tight. These taps must be made very nearly identical in order to avoid altering the optical power drawn off of the main rail channel as the state of the directional coupler is changed. Finally, the use of the organic polymer material to form those portions of the main rail and secondary side channels which do not require the electro-optic effect adds unnecessary optical loss to the system. Currently, channel waveguides formed of ion-exchanged glass can be made with far lower optical propagation losses than channel waveguides made of organic polymers. Thus for devices such as the page scanner, which require long propagation lengths, it is desirable to restrict the use of the higher propagation loss organic material to the modulator areas where the electro-optic effect is required.